1. Field of the Invention
The present invention relates to a microwave filter using a strip line or a micro-strip line, or more in particular to a microwave filter configuration with a pass-band frequency higher than a stop-band frequency and both the pass-band and stop-band frequencies limited in bandwidth.
2. Description of the Prior Art
In a mixer supplied with a radio frequency signal (f.sub.s in frequency) and a local oscillation signal (f.sub.l in frequency) different in frequency from the radio frequency signal for producing an intermediate frequency signal (f.sub.s -f.sub.l in frequency; f.sub.s &gt;f.sub.l) making up a frequency component representing the difference between the former two signals, a filter (hereinafter called the "signal-pass image-rejection filter) for passing the radio frequency signal without loss but stopping an image signal (with frequency f.sub.m = 2f.sub.l -f.sub.s) having a frequency (2f.sub.l -f.sub.s) twice the local oscillation signal (2f.sub.l) less the frequency (f.sub.s) of the radio frequency signal, is inserted in a main line for transmitting the radio frequency signal to a mixer diode. Further, a local band-pass filter (hereinafter called the "local BPF") for selectively passing a local oscillation signal alone is interposed between an input terminal for the local oscillation signal and the mixer diode. Upon application of a radio frequency signal and a local oscillation signal to a mixer diode making up a non-linear element, a side band or a high harmonic of mf.sub.s .+-.nf.sub.l (m, n: Integers) in frequency are generated. The waves of the image signal frequency f.sub.m and the sum frequency f.sub.s +f.sub.l in these spectra contain a radio frequency component. By returning the image signal, in particular, out of these signals to the mixer diode through a signal-pass image-rejection filter and mixing it with the local oscillation signal again, therefore, it is possible to produce a reconverted intermediate frequency signal and thereby to reduce the conversion loss of the mixer. Further, the signal-pass image-rejection filter is capable of preventing an interference wave signal having the same frequency as the image signal frequency from entering the frequency band of the intermediate frequency signal by way of the radio frequency signal input terminal.
Especially, a single-ended mixer using only one mixer diode has the performance thereof greatly affected by the manner in which the image signal generated in the mixer diode is processed. The impedance as viewed from a diode terminal is normally set to be reactive against the image signal frequency. A signal-pass image-rejection filter and a local BPF for rejecting an image signal thus constitute indispensable elements for configuring a single-end mixer. The signal-pass image-rejection filter is provided on or in coupling with a main line for transmitting a radio frequency signal to the mixer diode, and therefore the characteristics of the signal-pass image-rejection filter have a direct effect on the mixer performance. In other words, it is not too much to say that the mixer performance is determined by the characteristics of the signal-pass image-rejection filter.
The performance described below is required of such a signal-pass image-rejection filter.
(1) A minimum insertion loss against a radio frequency signal.
(2) Characteristics to reject an image signal sufficiently.
(3) A pass bandwidth and a rejection bandwidth required for a radio frequency signal and an image signal respectively.
(4) The more steep the out-of-band characteristics, the closer the frequencies of the radio frequency signal and the image signal to each other.
A conventional signal-pass image-rejection filter used with a mixer is disclosed in JP-A-63-10601. This signal-pass image-rejection filter is shown in FIG. 9.
In FIG. 9, an input terminal 1 and an output terminal 2 for a radio frequency signal are connected by a main line 3 configured of a strip line. Open-ended stubs 4, 5, 6 having lengths of l.sub.1, l.sub.2, l.sub.3 respectively at equal intervals of l.sub.0 sequentially are connected in shunt with the main line 3. The lengths l.sub.1, l.sub.2, l.sub.3 of the open-ended stubs 4, 5, 6 are selected as equal or near to one fourth of the wavelength of the image signal so that poles of attenuation are placed within or in the vicinity of the image signal band. The length, l.sub.1, l.sub.2, l.sub.3 and the intervals l.sub.0 of the open-ended stubs 4, 5, 6 are also determined in such a manner as to hold the relations of both l.sub.2 &lt;l.sub.1 &lt;l.sub.0 &lt; 2l.sub.2 and l.sub.2 &lt;l.sub.3 &lt;l.sub.0 2l.sub. 2 at the same time or the relations l.sub.2 &lt;l.sub.1 =l.sub.3 &lt;l.sub.0 &lt;2l.sub.2, while the length l.sub.0 is selected at a value about 1.5 times one fourth of the wavelength of the radio frequency signal. Numerals 7, 8 designate input and output lines connected to the input and output terminals 1 and 2 respectively.
The forementioned signal-pass image-rejection filter with the open-ended stubs 4, 5, 6 projected in the directions perpendicular to the main line 3 has disadvantages in that:
(1) The fact that the open-ended stubs 4, 5, 6 are mounted in the form projected in the directions perpendicular to the main line 3 easily causes radiation, thereby increasing an insertion loss within the pass band of a radio frequency signal.
(2) The open-ended stub 5 has poles of attenuation on high-frequency side as compared with the stubs 4, 6. If the characteristic impedance of the open-ended stub 5 is increased, a filter having a comparatively steep rise characteristic would be obtained. Since there is only one open-ended stub with poles of attenuation on high frequency side, however, it is impossible to produce a filter having a steep rise characteristic.
(3) In view of the fact that the open-ended stubs 4, 5, 6 are projected in the directions perpendicular to the main line, the filter is widened for an increased filter size.